The present invention is directed to a composite piston for ejecting a plastic mass from a generally cylindrically shaped receptacle. The composite piston includes a piston head arranged to contact the mass to be ejected and a platelike pressure component located on the opposite side of the piston head from the mass. The piston head and the pressure component form a chamber in communication with the space in the receptacle containing the mass with at least one opening in the piston head directed into the chamber and with a venting channel connecting the chamber with the ambient atmosphere.
Cylindrical receptacles with ejecting pistons displaceable within the receptacles have been used for a period of time for dispensing different sealing, filling and coating masses. The receptacles have been known for use with single or multi-component masses which harden after being dispensed from the receptacle or remain plastic to a certain degree. To avoid premature hardening and reaction of the mass in the receptacle during storage and transport, air must be completely evacuated from the receptacle after it is filled with the mass. As a result, air is evacuated by deforming the receptacle in cross section providing a gap-shaped channel through which air can escape when the piston is inserted. Furthermore, it is known to introduce a needle between the wall of the receptacle and the piston for forming venting channels.
In a known composite ejection piston formed of two parts, disclosed in Great Britain Pat. No. 2,072,755, the piston head has apertures communicating between the space containing the mass and a chamber located between the piston head and a pressure component. The chamber is in communication with the atmosphere through venting channels so that air can escape from the space holding the mass through the apertures into the chamber and then through the venting channels into the atmosphere. The apertures in the piston head are kept very small so that air can escape, however, the mass cannot pass through the apertures into the chamber. Apart from the problems concerning venting, it should be noted when used with multi-component masses, that the components are arranged in separate receptacles and are combined immediately before the ejection step in the desired quantity and mixture ratio required for the chemical reaction. Such receptacles are arranged in specialized apparatus for effecting a simultaneously advance of the pistons in the different receptacles which are coupled together by piston rods. For accurate maintenance of the required mixture ratio during the entire dispensing operation, the ejecting piston must be in a specific axial position at the commencement of the dispensing operation. If the required position is not maintained, at the start of the dispensing operation, only one component is ejected and the required mixture is not obtained whereby the mass cannot be used. The machines used for filling the component into the receptacles are, however, relatively inaccurate as far as the metered amount of the various masses is concerned, accordingly, the required precision of the initial positioning of the ejecting pistons cannot be achieved when the pistons are being inserted into the receptacles.